The present invention relates to measurement of cardiac output of a patient. More specifically, the present invention relates to an apparatus and method for non-invasive cardiac output measurement of a subject utilizing a respiratory gas analyzer employing a flow sensor, an oxygen sensor, and a pulse oximeter which are interconnected to measure the cardiac output of the subject.
U.S. Pat. No. 5,836,300 to Applicant discloses a respiratory gas analyzer for measuring the metabolic activity and the cardiac output of a subject including a bidirectional flow meter and a capnometer sensor interconnected by conduits and valving between a mouthpiece and a source of respiratory gases which can be a controlled source or the atmosphere. A computer receiving signals from the flow meter and the capnometer can then calculate the subject""s metabolic activity. When valving is shifted, a portion of the exhaled gases are stored in the conduit so that upon inhalation, the subject inhales a substantial portion of rebreathed gases. The computer can then calculate the patient""s cardiac output as a function of the changes in total carbon dioxide content of the exhaled gas before and after the valve is shifted from a direct input to a rebreathed position and the difference in end-tidal carbon dioxide between the two positions.
The cardiac output of a patient, that is the volume of blood ejected from the heart per unit time, is an important measured parameter in hospitalized patients. Currently, cardiac output is routinely measured by invasive techniques including thermal dilution using an indwelling pulmonary artery catheter. This technique has several disadvantages including the morbidity and mortality risks of placing an invasive intracardiac catheter, the infectious disease risks, significant expense and the fact that it provides an intermittent rather than a continuous measurement. A noninvasive, reusable cardiac output measurement device would substantially improve patient care and reduce hospital costs.
The partial rebreathing technique mentioned above is a known method for cardiac output measurement. As described in Kapec and Roy, xe2x80x9cThe Noninvasive Measurement of Cardiac Output Using Partial CO2 Rebreathing,xe2x80x9d IEEE Transactions on Biomedical Engineering, Vol. 35, No. 9, September 1988, pp. 653-659, the method utilizes well known Fick procedures, substituting carbon dioxide for oxygen, and employing a sufficiently short measurement period such that venous carbon dioxide levels and cardiac output can be assumed to remain substantially constant during the measurement.
U.S. Pat. No. 4,949,724 to Mahutte et al. discloses a method and apparatus for continuously monitoring cardiac output by utilizing a modified Fick equation. The Mahutte et al. patent replaces VO2 in the Fick equation by VCO2 divided by a constant representative of the gas exchange ratio of a patient in order to eliminate inaccuracies associated with monitoring the rate of uptake of oxygen.
In its original form, the Fick method of measuring cardiac output requires blood gas values for arterial and mixed venous blood as follows:       C    .    O    .    =            VO      2                      CaO        2            -              CvO        2            
where C.O. is cardiac output, VO2 is oxygen consumption, CaO2 is the arterial oxygen content, and CvO2 is the venous oxygen content.
By utilizing a respiratory analyzer with a fast-response oxygen sensor, the cardiac output can be determined based on the end-tidal oxygen concentration (EtO2). End-tidal oxygen concentration is the lowest value of oxygen concentration in breath. The end-tidal oxygen concentration approximates the pulmonary capillary oxygen concentration.
Alternatively, at different points in time, it is also true that       C    .    O    .    =                    VO                  2          ⁢                      (            1            )                                                CaO                      2            ⁢                          (              1              )                                      -                  CvO                      2            ⁢                          (              1              )                                            =                            VO                      2            ⁢                          (              2              )                                                            CaO                          2              ⁢                              (                2                )                                              -                      CvO                          2              ⁢                              (                2                )                                                        .      
If the oxygen concentration of the inspired gas is temporarily increased or decreased, the change in alveolar oxygen concentration will cause a transient uptake or release of oxygen across the pulmonary capillaries, thereby resulting in a change in the measured VO2 and arterial oxygen content (CaO2). If these parameters are measured during an interval of time less than the circulation time (i.e., less than approximately thirty-fifty seconds), then the venous oxygen content (CvO2) level remains essentially constant during this period and can be removed from the equation. Therefore, cardiac output can be determined based on the equation       C    .    O    .    =            Δ      ⁢              xe2x80x83            ⁢              VO        2                    Δ      ⁢              xe2x80x83            ⁢              CaO        2            
The use of these novel concepts in combination with the apparatus and method of the present invention therefore allows for the non-invasive measurement of cardiac output utilizing measurements of airway gases and arterial oxygen concentrations, both of which can be done by non-invasive techniques.
The present invention is accordingly directed toward an airway-based respiratory gas analyzer for measuring the cardiac output of a subject. In a preferred embodiment of the analyzer of the present invention, the analyzer includes a respiratory connector operative to be supported in contact with a subject so as to pass inhaled and exhaled gases as the subject breathes. A flow meter operatively connected to the respiratory connector generates electrical signals as a function of the volume of gases which pass therethrough and, in combination with the signals generated by an oxygen sensor, allows for the determination of oxygen consumption (VO2) by integrating the flow and oxygen concentration signals over an entire breath. The oxygen sensor can also provide for the measurement of end-tidal (EtO2) concentration. An oximeter provides measurements of the subject""s oxygen saturation. A computation unit receives the output signals from the flow sensor, oxygen sensor and oximeter and calculates the cardiac output based on the generated signals.
An alternative mechanism for performing measurements of the subject""s cardiac output includes the subject placing the mouthpiece of the analyzer into their mouth and breathing a first oxygen concentration for a first period of time. Typically, the source of respiratory gases is atmospheric air. As the subject breathes, oxygen consumption (VO2) is determined as the integral of the flow and oxygen concentration signals over the entire breath. The oximeter provides a measurement of the subject""s oxygen saturation which is utilized to calculate the subject""s arterial oxygen content. After obtaining the measurement of the oxygen consumption (VO2) and arterial oxygen content (CaO2) over the first time period, the oxygen blender is caused to provide an increase or decrease in the airway oxygen concentration of the subject for a second period of time which is less than the subject""s circulation time. The oxygen consumption (VO2) and arterial oxygen content (CaO2) are measured over this second time period on a breath-by-breath basis and are utilized in calculating the subject""s cardiac output.
According to one aspect of the present invention, there is provided a respiratory gas analyzer for measuring cardiac output of a subject, said analyzer comprising:
a respiratory connector operative to be supported in contact with a subject so as to pass inhaled and exhaled gases as the subject breathes;
a flow sensor operatively connected to said respiratory connector adapted to generate electrical signals as a function of the volume of gases which pass therethrough;
an oxygen sensor for sensing the concentration of oxygen in the inhaled and exhaled gases, to thereby enable a determination to be made of the oxygen consumed by the subject during each breath;
conduits interconnecting said respiratory connector, said flow meter, and said oxygen sensor;
an oximeter for enabling a determination to be made of the concentration of oxygen in the subject""s arterial blood; and
a computer for receiving output signals from said flow sensor, said oxygen sensor, and said oximeter to calculate the cardiac output of the subject without the need for sensing the concentration of oxygen in the subject""s venous blood.
According to one preferred embodiment of the invention described below, the analyzer is used in a two-measurement procedure, wherein the computer calculates the cardiac output (C.O.) of the subject according to the following equation:       C    .    O    .    =            Δ      ⁢              xe2x80x83            ⁢              VO        2                    Δ      ⁢              xe2x80x83            ⁢              CaO        2            
wherein: xcex94VO2 is the difference in said consumed oxygen in the two-measurement procedure, and xcex94CaO2 is the difference in said arterial oxygen in the two-measurement procedure;
and wherein: the two-measurement procedure involves:
(a) a first measurement of said consumed oxygen and said arterial oxygen during a first time interval, and
(b) a second measurement, following a change in the oxygen content of the inhaled air, during a second time interval having a duration less than the blood circulation time of the subject.
According to a second described preferred embodiment, the computer calculates the cardiac output (C.O.) of the subject computer calculates the cardiac output (C.O.) of the subject according to the following equation:       C    .    O    .    =            VO      2                      CaO        2            -              CvO        2            
wherein: VO2 is the oxygen consumed during a breath; CaO2 is the concentration of oxygen in the subject""s arterial blood; and CvO2 is the concentration of oxygen in the subject""s venous blood, which is assumed to be the same as the end-tidal oxygen concentration in the exhaled air.
Other features, advantages and applications of the present invention will be made apparent from the following detailed description of the preferred embodiments.